As is well known in the art, the demand for higher performance as well as higher degree of integration of semiconductor integrated circuits is ever increasing in recent years. For this reason, as a lithographic technique, in place of the photolithography employing ultra-violet rays of the prior art, efforts have been made to establish an ultra-fine pattern working technique by the use of ionizing radiation of higher energy with shorter wavelength, namely, an electron beam, soft X-rays, an ion beam, etc.
On the other hand, for enabling an ultra-fine lithography by such a change of radiation source, the resist material to be used must have corresponding characteristics. Generally speaking, the resist material to be used in ultra-fine lithography with the use of a high energy ionizing radiation is required to have the following characteristics:
(a) High sensitivity to the ionizing radiation; PA1 (b) High resolving power; PA1 (c) Capability of forming a uniform thin film; PA1 (d) Excellent resistance to dry etching so that dry etching which is essential for high density micropatternization may be applicable; and PA1 (e) Excellent developing characteristics. PA1 as the aromatic type: ##STR6## as the heterocyclic type: ##STR7##
In the prior art, a great number of resists sensitive to ionizing radiations have been developed to be used for the above purpose. These resists may be classified into the positive-type in which the irradiated portion disintegrates to be solubilized by irradiation of an ionizing radiation and the negative type in which the irradiated portion undergoes crosslinking to be insolubilized by irradiation of an ionizing radiation.
Of these, the positive-type resists entail difficulty in selection of developers because generally a narrow scope of developers has characteristics adapted therefor and also suffer from the drawback of weak resistance to dry etching. In contrast, most of the negative-type resists are superior in these respects to the positive-type ones.
Representative of the negative-type resists developed in the prior art are the polyglycidyl methacrylate type, glycidyl methacrylate-ethyl acrylate copolymer type, and unsaturated carboxylic acid-methacrylate copolymer type. However, these negative-type resists have some drawbacks in practical application and cannot be said to be satisfactory. For example, a glycidyl methacrylate type resist, while having high sensitivity, has a low resolution, practically as low as only about 2.0 .mu.m, due to generation of a large amount of scum at the brim portions of the drawn image pattern. Moreover, each of the above resists has low dry etching resistance (with too much or too fast reduction in film thickness during dry etching) and therefore has a disadvantage in that it is difficult to apply thereto a dry etching process, which is indispensable for high-density micropatternization.
In view of the state of the art as described above, one of the present inventors has already developed a novel negative-type resist sensitive to ionizing radiation which has high sensitivity and excellent dry etching resistance and is also capable of attaining a high degree of resolution. The negative-type resist sensitive to ionizing radiation comprises an acetalized polyvinyl alcohol polymer having a molecular weight of 10,000 to 1,000,000 represented by the formula: ##STR2## wherein R.sup.1 represents a residue of an aldehyde or a ketone; R.sup.2 a hydrogen atom, which may be partially substituted by an acetyl group; R.sup.3 naught or a monomeric unit copolymerizable with vinyl acetate; l, m, n integers indicating polymerization degrees.